1. Field of the Invention
The present invention relates to an electro-conductive roll such as a charging roll, a transfer roll, or a cleaning roll used in an electrophotographic process or an electrostatic recording process in an image forming apparatus, such as an electrophotographic copying machine or a printer; and an image forming apparatus using the electro-conductive roll.
2. Description of the Related Art
Recently in image forming apparatuses using electrophotographic systems, charging rolls have been widely used as members for uniformly charging the surface of a body to be charged, such as a photoreceptor. The charging roll, which has voltage applied thereto, comes in contact with the photoreceptor, and discharges electricity at a micro gap between the charging roll and the photoreceptor, thereby causing the surface of the photosensitive material to be charged. The resistance and shape of the above-described charging roll are strictly controlled so as to allow uniform charging of the photoreceptor.
In image forming apparatuses for electrophotography using charging rolls, foreign substances such as residual transfer toner, carrier, and paper dust, which adhere to the photoreceptor surface, come into a nip portion between the charging roll and the photoreceptor. Therefore, the foreign substances adhere to the surface of the charging roll and contaminate the charging roll. If foreign substances adhere to the surface of the charging roll, the portion of the charging roll with the foreign substances adhering thereto has a high resistance, thereby causing poor charging of the photoreceptor or abnormal discharging at the portion of the charging roll having the foreign substances. As a result, image defects such as white spots or color spots form in an image that has been developed and transferred. A charging roll that is continuously contaminated with foreign substances adhering thereto over a long period of time causes uneven resistance of the roll as a whole. Accordingly, irregular charging of the photoreceptor is apt to occur, and image defects such as unevenness of density, or white spots or color spots over the entire image are generated in the formed image.
Particularly, there has been an increasing demand for both enhancement of image quality and longer operating life in recent electrophotographic image forming apparatuses, and uneven charging that could not be detected by conventional methods causes image defects. Further, in cases in which a charging system employing only a direct current voltage application for the purpose of reducing the costs of an image forming apparatus, image defects caused by adhesion of foreign substances becomes even more conspicuous as compared with a conventional apparatus in which voltage is applied by superimposing an alternating current on a direct current to contemplate uniform charging. Moreover, brushes or rolls have been recently employed as a cleaning member of a photoreceptor in place of a blade, because of long-term use of the photoreceptor. However, when compared to using a blade, such brushes or rolls cause a larger amount of foreign substances, including toner, to penetrate into the charging roll.
In order to prevent the occurrence of image defects caused by the adhesion of foreign substances, there has been conventionally attempted: a method that reduces adhesion of foreign substances onto a charging roll; or a method of mounting a member for removing attached foreign substances. As the method that reduces adhesion of foreign substances to a charging roll, for example, an attempt has been made in which a surface layer is formed from high-polymer material having a dense molecular structure and having excellent releasing properties, which material is typified by so-called resin, thereby reducing adhesion of foreign substances to the surface of an elastic roll (charging roll) having a high adhesiveness (see Japanese Patent Application Laid-Open (JP-A) Nos. 58-194061 and 1-204081).
Further, as the method of mounting a member for removing an adhered foreign substances, an attempt has been made in which a roll or a pad (see JP-A No. 2-301777), a web (see JP-A No. 2-301779), or the like is made to abut against a charging roll, or a cleaning member is provided in the vicinity of a charging roll (see JP-A No. 58-194061), thereby removing foreign substances adhering to the surface of the charging roll.
Nonetheless, in the method in which the above-mentioned dense polymer material is used for a surface layer, such polymer material is generally hard, and therefore, there are cases in which poor charging may occur due to the adhesion of a small amount of foreign substances, or wear of the photoreceptor may be accelerated.
Further, in the method of mounting a member for removing adhered foreign substances as well, it was found that residual toner on the charging member surface cannot be sufficiently removed and foreign substances tend to gradually accumulate.
An attempt has also been made in which, based on the surface average roughness and film thickness of each of an elastic layer and a surface layer, adhesion of toner to the surface of a charging roll is reduced (JP-A No. 7-49605).
Additionally, the adhesion of foreign substances is a large problem not only for a charging roll, but also for members that are disposed around a photoreceptor in contact with the photoreceptor (for example, a transfer roll, a cleaning roll, and the like) since the durability of the members is deteriorated. Rolls that can fundamentally prevent or reduce adhesion of foreign substances or that do not cause deterioration of performance even if foreign substances adhere to the roll, have yet to be developed.